A local area network (LAN) protocol may consist of a discovery phase and a functional phase. The discovery phase is where a user on the local area network discovers the availability of a service or resource on the same local area network. The functional phase is where the service or resource is actually being used. An example of this is a shared printer. The discovery phase is where the printer broadcasts its availability to users within the LAN and where one of the users take notice. The functional phase is when the user actually prints to the shared printer.
For some protocols, the discovery phase is what limits the protocols to be LAN-only. A protocol's functional phase may actually be able to work across the Internet. But, because only users within the LAN can discover the service or resource, all actual uses become limited to the LAN.